Mobile bases with intended applications in robotics and industrial automation require a great deal of movement flexibility in order to be fully utilized. Current robot mobile bases often employ a “synchro-drive” mechanism—a complex set of gears and pulleys that constrains the wheels to steer and to translate simultaneously. Since steering and translation movement is fully decoupled in this system, “steering in place” is possible. (Contrast this maneuverability with that of a car.) While this allows a good deal of maneuverability, its mechanical complexity makes it difficult and expensive to manufacture. Additionally, this type of mobile base has limited movement due to its fixed orientation (i.e. its inability to rotate).
In general, a rigid body constrained to move in a plane (i.e. a mobile base moving on the floor) has three degrees of freedom (DOFs) such as, movement in the x direction, movement in the y direction and rotation. Combining these three DOFs results in movement in any direction while simultaneously rotating.
A holonomic mobile base, has the ability move in this manner and change its motion at any time. By constructing a mobile base out of wheels that each have two degrees of freedom (one for steering, one for translation) all three DOFs are possible under the proper control, and complex mechanisms found in synchro-drive mobile bases are no longer needed. Additionally, these 2-DOF wheels (2-DOFWs) can be easily integrated and manufactured as modular wheel assemblies. A complete description of one example of a robot base and modular wheel assembly is provided by U.S. patent application Ser. No. 09/134,241, by Holmberg et al., incorporated herein by reference.
It is also possible to increase the “caster” of a 2-DOFW by moving the translation axis behind the steering axis (FIG. 1) and making it resemble a caster wheel. Unlike a 2-DOFW wheel with no caster (i.e. intersecting steering and translation axes), a mobile base constructed with caster 2-DOFWs is fully holonomic under the proper control.
Various problems arise, however, when trying to control a mobile base constrained to three DOFs with more than three controllable DOFs (e.g. a base with four 2-DOFWs has eight DOFs). A base with this many DOFs under improper control will certainly result in undesired motion and motor axes that “fight” each other. (i.e. consider two wheels facing opposite directions playing tug-of-war.)
This in turn creates wheel slippage, increased tire wear, increased power consumption, and more frequent mechanical problems.
It is considered important in robotics to be able to accurately assess the motion of the mobile base either through direct measurement or through estimation. This motion estimation can also be “summed-up” over time to create a “dead reckoned” position estimate of the mobile base with respect to fixed coordinates, which is also useful for autonomous tasks such as navigation. That is, consider a robot that wishes to navigate to a location (room) of which it knows the x-y coordinates. Motion estimation is also responsible for proper control of the mobile base, as will be described below. A control algorithm which minimizes wheel slippage will also allow for maximum motion estimation accuracy.